1. Field of the Invention
The present invention relates to a sputtering method for forming a tantalum thin film electrode or electrode wiring for a semiconductor device.
2. Description of the Prior Art
In the past, in a liquid crystal display device, a display pattern is formed on a screen by selecting display picture elements arranged in a matrix. An active matrix driving method can be used for selecting display picture elements. According to this method, independent picture element electrodes are arranged so as to correspond to the respective picture elements, and switching elements are connected to the respective picture element electrodes, thereby performing a display. A liquid crystal display device using this method enables a display with a high contrast and can be put into practical use for TV, etc.
Examples of the switching elements for selectively driving picture element electrodes include TFT (thin film transistor) elements, MIM (metal-insulator -metal) elements, MOS (metal oxide semiconductor) transistor elements, diodes, and varistors. A voltage signal applied between the picture element electrodes and the electrodes facing thereto is switched (ON or OFF), whereby an optical modulation of liquid crystal existing therebetween is visualized as a display pattern.
In order to provide the liquid crystal display device of an active matrix driving system mentioned above with high precision, it is necessary to make the picture element electrodes as small as possible. In order to make the screen larger, for example, in the case where the switching elements are TFTs, it is necessary to make gate electrode wirings and source electrode wirings thinner and longer. Moreover, in the case where the TFT has inverted staggered structure, since the gate electrode wiring is formed mn an initial stage of the process for forming the TFT, it is required that the wiring be thin and have a sufficiently low resistance. Also, it is required that a material having a strong corrosion resistance be used for the wiring so as to withstand the subsequent process. In the past, as a material which satisfies these requirements, a metal material such as tantalum (Ta), chromium (Cr), or titanium (Ti) has been used. In particular, tantalum is mainly used, since tantalum has a strong acid resistance and a fine selfoxidized film can be obtained by an anodic oxidation method when tantalum is used as the material.
When a metallic thin film made of tantalum is formed by sputtering, argon is used as a sputtering gas in the conventional method, so that a tantalum structure becomes .beta.-Ta and its specific resistance is in the range of 170 .mu..OMEGA..cm to 200 .mu..OMEGA..cm. As a conventional well-known method for decreasing this specific resistance, there is a method for doping argon with nitrogen. It is confirmed that according to this method, the specific resistance can be minimized in the range of 70 .mu..OMEGA..cm to 100 .mu..OMEGA..cm.
However, the specific resistance at this level is not Sufficient. When the gate electrode wiring is made longer and thinner in a liquid crystal display device using the tantalum film as the electrode wiring for the purpose of realizing a large screen and high precision, the electrical resistance of the gate electrode wiring rapidly rises and the time constant of the gate electrode wiring increases. When the time constant of the gate electrode wiring increases, a rising of an ON signal at the TPT disposed at the and of the gate electrode wiring becomes slow. Also, a falling of the ON signal from-ON to OFF becomes slow. Because of this, the picture element at the end of the gate electrode wiring is not sufficiently charged during the writing of its signal, and the picture element takes in signals of the other picture elements. As a result, the liquid crystal display device cannot perform a normal display.
Accordingly, in order for the liquid crystal display device to perform a normal display, it is necessary to decrease the electrical resistance of the gate electrode wiring. As a means for this, a gate electrode wiring with a two-layered or three-layered structure using a material which can be readily formed into a thin film having a low specific resistance such as aluminum and molybdenum is considered.
However, in this kind of gate electrode wiring with a multi-layered structure, the process for manufacturing becomes complicated because of the necessity of an increase in the processes for forming films and the photolithography processes. In addition, the above-mentioned materials have a lower acid resistance compared with tantalum, so that the gate electrode wiring is likely to be damaged in the process after forming the TPT and a disconnection occurs, resulting in decreased reliability and yield.
In Japanese Laid-Open Patent Publication No. 62-205656, a method for decreasing the specific resistance of a thin film which is a material for the gate electrode wiring by mixing molybdenum with tantalum is proposed. According to this method, the specific resistance of a thin film becomes 40 .mu..OMEGA..cm.
However, when an alloy is formed by mixing molybdenum with tantalum, molybdenum contained in the film is eluted in the process for anodizing the gate electrode wiring, and an oxide film whose molecular orientation is denser than that of an oxide film obtained by anodizing tantalum cannot be obtained.